1. Field of the Invention
The present invention relates to a semiconductor light emitting element such as a light emitting diode (LED) for use in a display device, a signaling device, etc., a semiconductor laser element, or the like, and a display device and an optical information reproduction apparatus using the same; and a method for fabricating such semiconductor light emitting elements. Specifically, the present invention relates to a nitride semiconductor light emitting element having a hexagonal crystal structure; a display device and an optical information reproduction apparatus using the same; and a method for fabricating such a semiconductor light emitting element.
2. Description of the Related Art
Representative nitride semiconductor materials such as GaN, AlN, InN, and mixed crystals thereof have realized semiconductor light emitting elements such as a light emitting diode (LED) capable of emitting light in a range from visible light to ultraviolet light. These semiconductor light emitting elements have been applied to a display device, a signaling device, etc.
For example, a semiconductor light emitting element using a nitride-gallium semiconductor material includes a layered structure in which a GaN buffer layer, an n-type GaN contact layer, an n-type InGaN quantum well active layer, a p-type AlGaN sublimation preventing layer, and a p-type GaN contact layer are sequentially provided on the (0001) face of a sapphire substrate.
The layered structure has a mesa structure formed by removing portions of the p-type GaN contact layer, the p-type AlGaN sublimation preventing layer, the n-type InGaN quantum well active layer, and the n-type GaN contact layer so that a portion of the n-type GaN contact layer is exposed and to substantially parallel to the substrate surface. On the exposed surface of the n-type GaN contact layer, a negative electrode it provided, while a transmissive positive electrode is provided over a substantially entire upper surface of the p-type GaN contact layer in the mesa structure. Furthermore, a positive electrode pad is provided on a predetermined portion of the transmissive positive electrode.
Light generated in the n-type InGaN quantum well active layer is output upward through the transmissive positive electrode and, on the other hand, is horizontally output from side surfaces of the mesa structure.
Alternatively, a semiconductor laser element using a nitride-gallium semiconductor material includes a layered structure in which a GaN buffer layer, an n-type GaN contact layer, an n-type AlGaN cladding layer, an n-type GaN light guiding layer, an n-type InGaN quantum well active layer, a p-type AlGaN sublimation preventing layer, a p-type GaN light guiding layer, a p-type AlGaN cladding layer, and a p-type GaN contact layer are sequentially provided on the (0001) face of a sapphire substrate.
The layered structure has a mesa structure formed by removing portions of the p-type GaN contact layer, the p-type AlGaN cladding layer, the p-type GaN light guiding layer, the p-type AlGaN sublimation preventing layer, the n-type InGaN quantum well active layer, the n-type GaN light guiding layer, the n-type AlGaN cladding layer, and the n-type GaN contact layer so that a portion of the n-type GaN contact layer is exposed and is substantially parallel to a substrate surface. On the p-type GaN contact layer, an insulating layer is provided so that a portion of the upper surface of the p-type GaN contact layer is exposed. This insulating layer also covers the substantially entire mesa structure except a side surface of the mesa structure from which light is emitted, and covers an electrode face of the n-type GaN contact layer so that a portion thereof is exposed. On an upper surface of the p-type GaN contact layer that is exposed through the opening of the insulating layer, a positive electrode is provided, while a negative electrode is provided on the upper surface of the n-type GaN contact layer that is exposed through the opening of the insulating layer.
In the above-described nitride semiconductor light emitting element (light emitting diode), since the polarization of light emitted from side surfaces of a mesa structure is not controlled, the emission efficiency of such a semiconductor light emitting element is deficient.
Furthermore, in the conventional semiconductor laser element, the polarization of spontaneous emission light from end faces of an active layer is not controlled, and accordingly, the noise characteristic of the semiconductor laser element is unsatisfactory. That is, in the conventional semiconductor laser element, since the coupling coefficient xcex2sp of the spontaneous emission light to the laser oscillation mode is not controlled, the coherence length of the oscillated light is long, and when such a conventional element is used for a light source of an optical information reproduction apparatus for an optical disk or the like, the noise characteristic of the feedback induced noise may increase.
According to one aspect of the present invention, a semiconductor light emitting element includes: a first conductive type layer made of a nitride semiconductor which is deposited on a substrate, a quantum well active layer made of AlPGaQIn1xe2x88x92Pxe2x88x92QN (0xe2x89xa6P, 0xe2x89xa6Q, P+Q less than 1) which is deposited on the first conductive type layer, the quantum well active layer including a pair of barrier layers and a well layer interposed therebetween; and a second conductive type layer made of a nitride semiconductor which is deposited on the quantum well active layer, wherein spontaneous emission light from end faces of the quantum well active layer is polarized in a direction parallel to the substrate.
In one embodiment of the present inventions the well layer includes, in a mixed state, regions in which an In content is high and regions in which an in content is low; an average size of the regions in which an In content is high is 1 nm to 100 nm; and a density per unit area of the regions in which an In content is high to 1xc3x971011/cm2.
In another embodiment of the present invention, the substrate is a sapphire substrate; and the first conductive type layer is deposited on a (0001) face of the sapphire substrate.
In still another embodiment of the present invention, the substrate is a sapphire substrate; and the first conductive type layer is deposited on a face tilted from a (0001) face of the sapphire substrate by an angle equal to or greater than 0.050 and smaller than 0.2xc2x0.
In still another embodiment of the present invention, the substrate is a GaN substrate; and the first conductive type layer is deposited on a (0001) face of the GaN substrate.
In still another embodiment of the present invention, the substrate is a GaN substrate; and the first conductive type layer is deposited on a face tilted from a (0001) face of the GaN substrate by an angle equal to or greater than 0.05xc2x0 and smaller than 0.2xc2x0.
According to another aspect of the present invention, there to provided a display device using the semiconductor light emitting element of claim 1.
According to still another aspect of the present invention, there is provided an optical information reproduction device using the semiconductor light emitting element of claim 1.
According to still another aspect of the present invention, a method for fabricating the semiconductor light emitting element of claim 1 includes a step of growing crystal grains for the well layer, wherein either before or after the step of growing crystal grains for the well layer, the provision of III-group material is substantially stopped for 1 to 300 seconds.
In this specification, a quantum well and a quantum well active layer refer to a structure in which a semiconductor layer (well layer) having an average thickness of 20 nm or less is sandwiched with semiconductor layers (barrier layers) each having a bandgap larger than that of the well layer.
Furthermore, in this specification, regions within the quantum well active layer in which the in content is high (high InN mixed crystal ratio regions) refers to regions in which the in content is higher by 5% or more than the average content within any 24 nmxc3x9724 nm square plane in the well layer which is parallel to the substrate surface. On the other hand, regions within the quantum well active layer in which the In content is low (low InN mixed crystal ratio regions) refers to regions in which the in content is lower by 5% or more than the average content within any 24 nmxc3x9724 nm square plane in the well layer which is parallel to the substrate surface.
Furthermore, in the present specification, the thickness of the well layer in the quantum well active layer is represented by the average value of the thickness, for a 24 nm length along any direction parallel to the substrate surface within a certain cross-section of the well layer, which is perpendicular to the substrate surface. This average value is referred to as the average thickness of the well layer.
Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor light emitting element having an improved light emission efficiency and satisfactory noise characteristics, and a fabrication method thereof; and (2) providing a display device and an optical information reproduction apparatus using such a semiconductor light emitting element.